Natural gas has often been touted as an energy source that offers a cleaner-burning "bridge" away from other fossil fuels like coal and oil. However, recent studies of leakage from gas fields and production facilities suggest that natural gas use may be far more dangerous to the planet's climate than expected -- with potentially serious ramifications for solar thermal generating plants as well.

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Natural gas is methane, produced deep underground by the decomposition of buried organic matter. When burned in a modern turbine, this methane can create electrical power with far lower greenhouse gas emissions per kilowatt-hour than other fossil fuels, especially coal.

The problem is that methane is itself a powerful greenhouse gas, rated by the Intergovernmental Panel on Climate Change as posing about 21 times the threat to the earth's climate as carbon dioxide per ton. In other words, if natural gas escapes to the atmosphere without being burned -- and thus turned into carbon dioxide and water vapor -- then it worsens the planet's climate plight.

Studies have suggested that despite methane's strong greenhouse gas properties, switching from burning coal to gas to generate electricity is nonetheless a boon to the climate as long as no more than 3.2 percent of natural gas used leaks to the atmosphere during drilling, transportation, storage or use. More than 3.2 percent leaking to the atmosphere, on the other hand, undoes much of the benefit from replacing coal as our power-generating fossil fuel of choice.

The report was controversial, and some scientists questioned the methodology used. Then in December, the NOAA-UC Boulder team announced that it had found leakage rates of nine percent from a gas field in Utah's Uinta Basin, a major gas-producing region. In other words, the gas field was losing almost a tenth of its gas to the atmosphere.

Whether gas fields elsewhere in the U.S. suffer similarly high leakage rates is unknown, and the scientists urge more study. The potential for climate damage is daunting, however. In 2008, the Uinta Basin produced 299.3 billion cubic feet of natural gas. Nine percent of that would be nearly 27 billion cubic feet lost to the atmosphere, with a greenhouse gas power equivalent to more than 11 million tons of CO2.

If you're a typical, somewhat car-happy Californian, it would take you more than 900,000 years to emit an equivalent amount of greenhouse gases.

It's no wonder climate activists are concerned lately about the explosion of natural gas extraction enabled by the controversial process of hydraulic fracturing -- "fracking '' -- which is thought to be one possible reason for the higher-than-expected leakage rates in Colorado and Utah. But as a result of fracking, natural gas is now much cheaper than it has been.

And as a partial result of natural gas' recent low prices, some California solar plants have included a natural gas component to help generate power when the sun's not out. This is especially true of solar thermal power plants. Solar thermal plants generally use a heat-conducting "transfer fluid," heated by concentrated solar energy, to run turbines. One the sun goes down, that transfer fluid starts to cool. It can take some time the next morening for the sun to reheat that fluid to the point where it's hot enough to move the turbines again.

For this reason, a number of solar thermal plants include a natural-gas-fired element to keep that fluid hot, to start things going a bit earlier in the morning and to even out production during cloudy weather.

The Ivanpah Solar Electric Generating System, for instance, now being built by BrightSource in the California desert near the Mojave National Preserve, includes natural gas-fired boilers for each of its planned three power towers. According to the California Energy Commission, those gas-fired boilers burn about 833 million BTUs of natural gas each hour, and are expected to be used for an hour every day. 833 million BTUs is a bit more than 810,000 cubic feet of natural gas. If we assume that those boilers are indeed used only one hour per day, then over the plant's claimed 25-year lifespan the Ivanpah complex will burn about 7.4 billion cubic feet of natural gas.

That's small potatoes compared to a wholly gas-fired plant of similar capacity. But if leakage rates for the fields supplying Ivanpah are similar to those found in the Uinta Basin, then the Ivanpah plant has to add close to 670 million cubic feet of leaked methane to its lifetime greenhouse footprint.

BrightSource has touted the greenhouse gas reductions from its Ivanpah plant as the equivalent of removing 90,000 cars from the road, but that 670 million cubic feet of potential methane leakage over the plant's lifetime would add 59,880 of those cars back, at least for a year.

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